1. Field of the invention
The present invention relates to a birefringent plate. The birefringent plate is utilized as an optically functional device, such as a 1/4-wavelength phase-difference plate and a 1/2-wavelength phase-difference plate, in a pick-up for a CD-ROM player or a DVD player.
2. Description of the Related Art
A birefringent plate has been used conventionally. In the birefringent plate, an obliquely deposited film is formed on a transparent glass substrate, and is made from a dielectric material, such as oxide, sulfide and fluoride, which is transparent in the visible light to the near-infrared region. The "obliquely deposition" herein means a method to form a film on a substrate surface which is inclined with respect to the flying direction of a depositing material. The structure of the obliquely deposited film is observed as a columnar structure which has an assembly of fine columns. The fine columns have circular cross-sections, and are inclined at a definite predetermined angle with respect to the surface of the substrate. The density of the columns exhibits anisotropy in a plane, and the refractive index exhibits anisotropy in a plane. As a result, the obliquely deposited film effects the birefringence. By utilizing the birefringence of the obliquely deposited film, the birefringent plate is applied to optical devices, such as a 1/4-wavelength phase-difference plate and a 1/2-wavelength phase-difference plate.
One of the problems associated with the obliquely-deposited birefringent plate is that the phase difference varies because of a change of the refractive index in accordance with an amount of the water which is occluded in the obliquely deposited film. The birefringent refractive index of the obliquely deposited film depends on the packed rate of the columnar structure and the refractive index of the columns and the substance occupying the spaces, etc. It is known that the birefringent refractive index of the obliquely deposited film nearly monotonously decreases when the refractive index of the substance occupying the spaces enlarges. For example, when all of the substance occupying the spaces is water whose refractive index n is 1.33 (i.e., n=1.33), the birefringent refractive index is reduced by half with respect to the case where all of the substance occupying the spaces is air whose refractive index n is 1.0 (i.e., n=1.0).
Tantalum pentoxide (Ta.sub.2 O.sub.5) is one of the film materials which are often used for the obliquely deposited birefringent plate. Immediately after the preparation, a Ta.sub.2 O.sub.5 obliquely deposited film is reduced to exhibit liver brown, but is turned into transparent by carrying out a bleaching (oxidation) treatment. The bleaching treatment is carried out in a dry-air atmosphere whose temperature is 90.degree. C. or less, or in a highly-humid atmosphere whose relative humidity is 85% or less. Thereafter, the Ta.sub.2 O.sub.5 obliquely deposited film is left in a constant temperature-and-humidity atmosphere, for example, at 25.degree. C. and in a relative humidity of 60%.
As earlier mentioned, the obliquely deposited film is composed of the columnar structure of a low density (a packed rate of from 70% to 80%), and has spaces in an amount of from 20% to 30% by volume. In the bleaching step and under the constant temperature-and-humidity atmosphere, a large amount of water is adsorbed and occluded in the spaces of the obliquely deposited film. This phenomenon was confirmed by an infrared spectroscopy analysis. The amount of the occluded water is saturated when water is adsorbed in all of the spaces of the columnar structure. The amount of the occluded water in the obliquely deposited film varies in accordance with the temperature. When the obliquely deposited film is exposed to an atmosphere of 100.degree. C. or more, the occluded water evaporates. Accordingly, air is the main component occupying the spaces. When the temperature is decreased to room temperature, the obliquely deposited film retrieves the water vapor in air. Then, the amount of the occluded water is recovered to the original amount.
Thus, the occluded water comes in and out of the obliquely deposited film in accordance with the temperature. Consequently, when the amount of the occluded water varies in the obliquely deposited film in accordance with the temperature, the refractive index of the substance occupying the spaces changes. As a result, the birefringent refractive index of the obliquely deposited film varies so that the phase difference fluctuates.
Japanese Unexamined Patent Publication (KOKAI) No. 1-312,507 discloses a countermeasure to the problem. For instance, a transparent resin, such as an epoxy resin, is injected into the spaces in the columnar structure, thereby improving the temperature and moisture characteristics of the obliquely deposited film. In this method, however, since the injected resin exhibits a high refractive index, the birefringent refractive index of the resulting birefringent plate decreases greatly. As a result, it is necessary to thicken the obliquely deposited film in order to obtain desired characteristics.
Another one of the problems associated with the obliquely-deposited birefringent plate is that the phase difference fluctuates depending on the measurement positions within the identical substrate when the phase difference was examined with a coherent light source, for example, a laser beam despite the fact that the characteristics of the obliquely deposited film, such as the film thickness and the birefringent refractive index, are constant. This phenomenon does not occur when an incoherent light is spectroscopically separated and is used as a light source. This is an adverse characteristic when a monochromatic light, such as a laser beam, is used as a light source.
Usually, the distribution of the phase difference is periodic. For instance, in a glass substrate of 5 cm.times.5 cm in size, the phase difference fluctuates by 10 deg. or more in a certain case. When the obliquely-deposited birefringent plate is used as a 1/4-wavelength phase-difference plate or a 1/2-wavelength phase-difference plate, the fluctuation of the phase difference results in a decreased yield of product and a remarkably increased cost because it is necessary to inspect every single device of 3 mm.times.3 mm in size.
Since the distribution of the phase difference occurs periodically, one of the causes of the fluctuation of the phase difference is considered to be the uneven thickness-wise inclination of the glass substrate. Normally, as for a substrate used in the obliquely deposited birefringent plate, a flat glass substrate having a surface roughness of 20 .ANG. or less is used. When the irregularity is large on the surfaces, the resulting obliquely deposited film whitens because the diameters of the columns enlarge. Regarding the flat glass substrate, a float glass substrate and a polished glass substrate have been known. However, in these glass substrates as well, there arise uneven thickness-wise inclinations which are equivalent to the wavelength of a laser beam.
Generally, it is possible to reduce the flatness of the surfaces of a glass substrate sufficiently less than the wavelength of a light. However, it is difficult to keep the parallelism between the front and opposite surfaces which is the thickness of a glass substrate of a large area. Whilst, it is relatively easy to make the thickness of the obliquely deposited film even by optimizing the geometric arrangement of the deposition. Accordingly, as illustrated in FIG. 8, the cross-section of an actual obliquely deposited film is made in such a manner that an obliquely deposited film of an even thickness is formed on a glass substrate whose size is a couple of centimeters and which has a thickness inclination nearly equal to the wavelength of a light.
When a highly coherent light, such as a laser beam, enters the thus constructed obliquely deposited birefringent film, the light interference occurs not only in the obliquely deposited film but also between the opposite surface of the glass substrate and the surface of the obliquely deposited film. The phase-difference distribution is caused in a plane of the birefringent plate by the light interference between the opposite surface of the glass substrate and the surface of the obliquely deposited film.
Namely, as illustrated in FIG. 8, the actually observed light is the superimposition of a beam 1 and a beam 2 which pass in the obliquely deposited film exhibiting the birefringence. For example, the beam 1 influences the beam 2 differently in the case where the effective optical thickness, the summed thickness of the obliquely deposited film and the glass substrate, is (2 m+1).lambda./2 from in the case where it is (2 m+1).lambda./4.
Accordingly, when the glass substrate has a definite predetermined thickness inclination as illustrated in FIG. 8, the phase difference varies periodically as the phase difference is measured along the direction of the inclination. Likewise, when the glass substrate has irregular thicknesses, the phase-difference distribution is observed in a plane.
The phase difference varies as aforementioned when the following 4 conditions are overlapped: 1) when the obliquely deposited film exhibits a high refractive index; 2) when the glass substrate has the periodic thickness inclination; 3) when the incident light reflects at the opposite surface of the glass substrate; and 4) when the measuring light source is a highly coherent laser beam. If a birefringent plate is constituted by an obliquely deposited film which is made by using SiO.sub.2 or MgF.sub.2 exhibiting a low refractive index, the problem can be solved to a certain extent. However, SiO.sub.2 and MgF.sub.2 cannot be used in optically functional component parts, such as a 1/4-wavelength phase-difference plate, because they exhibit a small birefringent refractive index and whiten. On the other hand, a Ta.sub.2 O.sub.5 obliquely deposited film does not whiten, but exhibits a large birefringent refractive index. However, Ta.sub.2 O.sub.5 exhibits a refractive index of 1.86, and produces an interference reflection of 10.7%. Moreover, it is difficult to eliminate the periodic thickness inclination of the glass substrate by ordinary methods for forming the glass substrate.